Lanthanide ions complexation by uronic acids

Article abstract of DOI:10.1016/S0022-2860(00)00942-X

FT-IR spectra of the crystalline D(+)-galacturonic and D-glucuronic acids, as well as their lanthanum salts have been recorded, assigned and discussed in terms of the structure of the investigated complexes.

Full text of DOI:10.1016/S0022-2860(00)00942-X

Journal of Molecular Structure 563±564 +2001) 587±593
www.elsevier.nl/locate/molstruc
Lanthanide ions complexation by uronic acids
b
b
L. Fuksa,b, , D. Filipiuk , W. Lewandowski
*
aInstitute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland
bBialystok Technical University, Zamenhofa 29, 15-435 Bialystok, Poland
Received 31 August 2000; revised 17 November 2000; accepted 17 November 2000
Abstract
FT-IR spectra of the crystalline d+1)-galacturonic and d-glucuronic acids, as well as their lanthanum salts have been
recorded, assigned and discussed in terms of the structure of the investigated complexes. q 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Lanthanide complexes; Natural chelates; Uronates; Uronic acids
1. Introduction
the effectiveness of these chelates as toxic metal
decontaminants +mobilizing agents) have remarkably
improved. Several new arti®cial chelates, which are
signi®cantly more effective or easier to administer,
have been proposed. Aprolonged treatment cannot,
however, be prescribed routinely because the chelate
may also cause unwanted side reactions, resulting for
example in kidney injury [1].
Clinical tests have shown that several natural poly-
saccharides, e.g. alginic acid +a straight chain, poly-
uronic acid consisting of anhydro a-d-mannuronic
acid and l-guluronic acid) or pectines +partially meth-
oxylated polygalacturonic acid), applied for the
detoxi®cation of living organisms from various
heavy metals induce an increased excretion of lead
[2] or mercury [3±5]. Furthermore, sodium alginate
incorporated into the diet of mice inducted an increase
of 90Sr, 133Ba and 226Ra radioisotope concentrations in
blood as well as a decrease of their deposition in the
skeleton [5]. In the marine environment, in turn,
macroalgae have been used as bioindicators for ®ssion
products or transuranium elements following fallout
from nuclear detonation tests and releases from
nuclear facilities [6,7].
The development of new procedures for the human
body detoxi®cation from various heavy metals is of
great importance because of growing industrial and
commercial use of their toxic compounds followed
by their increasing release into the environment. In
traditional medicine, the most popular chelating
agents ethylenediaminetetraacetic acid +EDTA) and
diethylenetriaminepentaacetic acid +DTPA) have
several carboxylic groups, which are ionized under
physiological conditions. Therefore, the side effect
of their application is the non-speci®c binding of the
majority of cations followed by the unintentional
removal of such important metals as calcium or potas-
sium from the body.
During recent years, the synthesis of arti®cial
chelating agents with large stability constants and
the delineation of the other factors, which determine
* Corresponding author. Institute of Nuclear Chemistry and
Technology, 16 Dorodna Street, 03-195 Warsaw, Poland.
Tel.: 148-22-811-2735; fax: 148-22-811-1532.
E-mail address: lfuks@orange.ichtj.waw.pl +L. Fuks).
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-2860+00)00942-X

588
L. Fuks et al. / Journal of Molecular Structure 563±564 62001) 587±593
Lanthanide ions +called rare earth elements) are
of the reaction, a white solid was formed. Finally, the
suspension was ®ltered, washed several times with
anhydrous acetic acid, and dried under reduced pres-
sure +0.1 atm; 708C; 6 h).
found in living organisms in trace amounts only,
measured in ppb units [8]. Because of this, they
are not considered to play a signi®cant biological
role [9,10]. However, similarity in the ionic radii
of the trivalent REEs with those of calcium and
sodium, in the charge density +Z/r) with magne-
sium or zinc, followed by the possibility of mutual
isomorphous replacement, combined with the
unique magnetic and spectroscopic properties of
the REEs make them very informative substitution
probes in the various biological and medical
examinations [11,12]. Finally, because of their
chemical similarity, REEs may also act as substi-
tution probes for the actinides Ð especially for
those radioactive elements available only in
submicroquantities.
The purpose of the present work is to examine
the interaction between d-glucuronic and d+1)-
galacturonic acids, which can be considered as
the basic parts of the polyuronides and the triva-
lent REE cations. Acomplete study of these
complexes has not been made until now and the
fragmentary results reported by different authors
are con¯icting even in the compound composition
[13±17]. In the present paper, we report the
conclusions drawn from the FT-IR studies,
performed as an extension of the absorption spec-
troscopic investigations of the selected trivalent
REE cation complexes with d-glucuronic and
d+1)-galacturonic ligands [18].
Lanthanum±sugar salts [20]: Asolution of the
hydrated lanthanum acetate +1 mmol in 20 ml of
water; pH ˆ 4:5) was added to hot uronic acid
solution +2 mmol in 20 ml of water; the same
pH) and warmed gently for 20 min at 808C. The
resulting solution was cooled down to room
temperature and ethanol was used to extract the
salt precipitate. After the ethanol±water solution
evaporation, lanthanum uronate crystals were
washed several times with ethanol and dried in
vacuum +0.05 atm; 308C; 8 h). The molecular
weight of the obtained salts +520.5 and 526.9 for
the galacturonate and glucuronate, respectively)
was determined by the EDTAtitration of the
investigated salt solutions in the anhydrous
ethanol. Lanthanum content in the investigated
samples was determined by means of the instru-
mental neutron activation analysis +INAA). Anal.:
calcd 26.3%; found 26:16 ^ 5:77% and 26:41 ^
4:98% +for the glucuronate and galacturonate,
respectively). It means that both lanthanum
glucuronate and galacturonate are salts of the
average 1:2 metal/ligand composition.
2.2. Measurements
FT-IR spectra were recorded using the Perkin±
Elmer System 2000 spectrometer within the 4000±
150 cm21 range with the resolution of 2 cm21. Spectra
of the solid samples were registered twice:
2. Experimental
² after the pelletization in KBr +about 1% of the
investigated species);
2.1. Chemicals
² with the ATR technique.
Lanthanum chloride anhydrous +purum; CAS:
10099-58-8), acetic acid +purum; CAS: 64-19-7), d-
glucuronic acid +purum; CAS: 6556-12-3), and d+1)-
galacturonic acid monohydrate +puriss; CAS: 685-73-4)
were supplied from Fluka. Water used was deionized
and doubly distilled in a quartz apparatus.
No signi®cant differences in both the series of spectra
were detected. This means that alterations of the
spectra originating from the potassium±uronic acid
interactions are negligible.
Lanthanum acetate [19]: Anhydrous lanthanum
acetate, La+CH3COO)3, was prepared by heating
lanthanum chloride in acetic acid under re¯ux until
no additional hydrogen chloride could be detected at
the top of the installation. In the course of the progress
3. Results and spectra assignment
The FT-IR spectra of d-glucuronic acid, d-galac-
turonic acid and their lanthanum salts will be

L. Fuks et al. / Journal of Molecular Structure 563±564 62001) 587±593
589
Table 1
Vibrational assignments made for the investigated species. Relevant bibliography is given in the discussion part of the paper. +Bands: s, strong;
sh, shoulder; m, medium; w, weak. Assignments: n, stretching; d, bending; t, internal rotation; r, rocking; w, wagging)
Assignments
Galacturonic acid
La±galacturonate
about 3445s
Glucuronic acid
La±glucuronate
about 3340s
n+OH) +strong hydrogen bond)
n+OH) +weak hydrogen bond)
n+CH) +a-anomer)
n+CH) +b-anomer)
Overtones
3447s, 3361s
3324s, 3284sh, 2981m, 2955m
2921m
3444sh, 3405vs
3281s, 3231sh, 3166s
2925s
2863m, 2839m
2752m, 2709m
2613m, 2547m
1712s
2892s, 2839m
2764m, 2700m, 2645m
2593m, 2559m
1705
Overtones
n+HOCO)
d+H2O)
n+CyO) antisymmetric
1643m
1596vs
1590vs
1510sh
1538sh
1484sh
1460m
d+COH) 1 d+CCH)
d+CCH) 1 d+COH)
n+OC) symmetric
1438m
1425s
1433m
1413m
1418sh
d+CCH) 1 d+COH)
d+COH) 1 d+OCH)
d+COH) 1 d+OCH)
d+CCH) 1 d+COH)
d+COH) 1 d+CCH)
n+CC) 1 n+CO)
1367s
1355m
1366s
1348m
1316sh
1149m
1349m
1299m
1327m, 1302m, 1283m
1243sh
1320w, 1289sh
1260s
1233sh, 1215m
1154s
1226s, 1204m
1158s
1204w
1150sh
1111sh
n+CO) 1 n+COC)
1119sh
1126m
n+CO) 1 n+COC) 1 d+CCO)
n+CO) 1 n+CCC)
n+CO) 1 n+CCO) 1 d+CC)
1103sh
1088vs, 1076sh
1054m, 1025vs
1094vs, 1071s
1055s
1047s
1047s
947m
1026s
1016sh
n+CO) 1 d+CCH)
n+CO) 1 d+CCO)
n+CO) 1 n+CCH)
d+CC) 1 n+CO)
1008sh
970m
940m
908w
863w
822s
944sh
908sh
896w
917w, 913w
878w
d+CH) +b-anomer)
d+CH) +a-anomer)
t+CCO) 1d+CCO) 1 d+CCH)
d+CCO) 1 d+OCO) 1 t+CO)
dr+H2O)
875sh
817m, 807m
802m
776m
779w, 767w, 752w
715s
767m
692m
d+CCO) 1 t+CCO)
dw+H2O)
655m
675m
633s
778m
672m
629m
546m
474m
408w
617m, 612m
about 520m
498m
617m
t+CO) 1 d+CCO)
d+CCO) 1 t+CO)
d+CCO) 1 d+CCH)
515m
473m
427w
587s, about 530m
457m
443w, 405w
415w, 403w
discussed in separated parts concerning:
salts are substantially different from those of the
corresponding free acids. The spectra of these salts
are covered by the strong and broad absorption
bands related mainly to the OH, CO22, CO stretching
and COH bending vibrations, which overlap several
other ®ne and sharp sugar absorption bands. The coor-
dination involves two uronic acid anions, each
binding through C+6) and C+60) carboxyl oxygen
atoms and C+5) ring oxygen atom, as well as several
+a) sugar OH stretching vibrations and binding
modes;
+b) carboxyl coordination;
+c) ribose ring vibrational frequencies;
+d) coordinated water molecules.
The infrared spectra of the lanthanum±uronic acid

590
L. Fuks et al. / Journal of Molecular Structure 563±564 62001) 587±593
Fig. 1. FT-IR spectra of the lanthanum glucuronate +upper spectrum) and glucuronic acid +lower spectrum).
water molecules resulting in the usual coordination
geometry around the REE cation [18]. The main
features of the spectra relevant to the following
discussion are given in Table 1. Simultaneously in
Fig. 1, we graphically present the changes in the
glucuronic acid spectra, which occur in the course
of the lanthanum salt formation. For the galacturonic
acid±lanthanum galacturonate pair, the differences of
the spectra appear to be similar.
cular hydrogen-bonding network of the acids [25],
while those of the frequencies lower than 3350 cm21
to the intramolecular hydrogen-bonding system of the
sugar [26]. Because it can be assumed that such
absorption bands do not appear in the spectra of a-
arabinose and its metal complexes, the intramolecular
hydrogen-bonding system is caused by the presence of
the carboxylic group [27,28]. The CH stretching
vibrations of the free ligand, placed at 2950±
2830 cm21, appeared as moderate or weak absorption
bands. Upon the metalation of the free sugar acids
they disappear, covered by very broad bands charac-
teristic for hydrogen-bonded OH bonds.
3.1. 4000±2500 cm21 region
In this region, the free sugar shows nine strong or
medium, sometimes broad, absorption bands. The ®rst
six can be related to the OH stretching vibrations and
the last three to the CH stretching vibrational frequen-
cies [23,24]. Bands of the frequencies higher than
3350 cm21 may be attributed to the strong intermole-
3.2. 2500±150 cm21 region
3.2.1. Carboxylic group bonding modes
The strong absorption bands at 1710 cm21 of the

L. Fuks et al. / Journal of Molecular Structure 563±564 62001) 587±593
591
free acids, assigned to the HOCO stretching vibration
[25], shift toward the lower frequency and split into
two bands +at about 1590 and 1430 cm21, respec-
tively) in the spectra of the lanthanum salts. The latter
bands may be assigned to CO22 antisymmetric and
symmetric stretching-vibrations, respectively [29].
The splitting and shifting of the CO stretching vibration
of the sugar acids upon salt formation may be indicative
of the participation of the carboxylic O+6) and O+60)
oxygen atoms in metal±sugar bonding [29]. The extent
of CO22 band splitting, as well as the symmetric and
antisymmetric stretching frequencies +separation about
150 cm21) indicate chelation or bridging of the acid
anions in these metal±sugar salts [30].
cule [29] appear as moderate absorption bands at
1643 cm21 only in the spectrum of the galacturonic
acid. Absorption bands of bonded water molecules,
generally appearing at about 3400 cm21 in the
lanthanum hydrated salts of both the uronic acids,
interfere with the strong hydrogen-bonding system
of the uronic acid residue in these salts. Because of
the presence of many absorption bands at 800±
400 cm21, it was not possible to relate any absorption
band to the wagging and rocking motions of the coor-
dinated water molecules [29].
3.2.4. Conformations of uronic acid
Structural studies have indicated that metalation of
the sugar moieties can signi®cantly affect the sugar
conformations [30±32]. Calcium ion was found to
show a certain af®nity toward different sugar confor-
mations. On the basis of infrared spectroscopy, it has
been demonstrated that the alkali metal cations favor
the b-anomer conformation [21], whereas the a-
anomer is stabilized by alkaline earth metal ions
[22,33]. These spectroscopic results are consistent
with the X-ray structural analysis obtained for the
above-mentioned metal±sugar salts [30,34,35]. The
characteristic infrared absorption band for the b-
anomer was observed at about 870 cm21 in the spectra
of the alkali metal±glucuronate salts [24]. The
presence of an absorption band at about 820 cm21 in
the spectra of the alkaline earth metal±sugar
complexes was attributed, in turn, to the a-anomer
conformation [22,33]. In the present work, the
presence of a weak absorption band at about
870 cm21 in the spectrum of the free galacturonic
acid as well as of both the investigated salts may be
related to the b-anomer conformation. The appear-
ance of the band at 822 cm21 in the spectrum of the
free galacturonic acid suggests that commercial
samples of the acid must be treated as mixtures of
both anomers.
3.2.2. Ring COC and COH bonding modes
Drastic changes of the spectra in the region of
2500±150 cm21 occurred upon the sugar acid metala-
tion. The atomic motions of the sugar ring-system are
strongly coupled, so the vibrational frequencies are
spread over the entire molecule. As a result, individual
assignments for each absorption band are dif®cult and
uncertain. The most important assignments and
changes observed upon lanthanum salt formation are
summarized below:
² Seven/eight absorption bands of moderate intensity
+appearing in the range of 1365±1205 cm21 in the
spectra of the free acid) are assigned to the bending
motions of CCH and COH [23,24]. These bands
shift toward lower frequency and signi®cantly
decrease in intensity, or even disappear, in the
spectra of the hydrated lanthanum salts. One can
suppose that the shift of the COH bending vibration
may be caused by the participation of the OH
groups in metal±uronic acid interactions.
² Six/seven strong absorption bands at 1160±
1015 cm21 in the spectra of ligands can be assigned
to CO stretching frequencies [23,24]. They appear
in the spectra of the lanthanum salts as two absorp-
tion bands at about 1150 and 1047 cm21
.
² Small decrease of the CO frequencies points to the
participation of the ring C+5) oxygen atoms of both
uronic acid residues in the metal±ligand interaction.
4. Conclusions
In order to assess the potential of naturally occur-
ring substances in the treatment of heavy metal intox-
ication, the interaction between d+1)-galacturonic
and d-glucuronic acids with the selected trivalent
lanthanide ions has been already studied in aqueous
3.2.3. Coordinated water molecule modes
Bending vibrations of the coordinated water mole-

592
L. Fuks et al. / Journal of Molecular Structure 563±564 62001) 587±593
[2] O. Vanderbroght +Ed.), Reprints on Prevention of Uptake on
Decorporation of Radioactive Heavy Alkaline Earth Metals in
Man, Mice, Swine, Belgian Nuclear Centre Ð Mol Report,
1981.
[3] P.A. Chaika, Gig. Tr. Prof. Zabol. 10 +1966) 47.
[4] S. Stanchev, Kh. Krachanov, M. Popova, N. Kirchev, M.
Marchev, Z. Ges. Hyg. 25 +1979) 585.
[5] I.M. Trakhtenberg, Yu.N. Talakin, G.E. Leskova, V.N.
Kakovskaya, N.V. Gridneva, Gig. Tr. Prof. Zabol. 24 +1980)
33.
[6] R.J. Pentreath, Impacts of radionuclide releases into the
marine environment, Proc. IAEA Conf., Vienna, 6±10 Oct.
1981, p. 241.
[7] H. Dahlgaard, A. Arkrog, L. Hakkstadius, E. Holm, J.
Â
Rioseco, Rapp. P.V. Reunion Cons. Int. Expl. Mer. 186
+1986) 70.
[8] G.V. Iyengar, W.E. Kollmer, H.J. Bowen, The Elemental
Composition of Human Tissues and Body Fluids, Verlag
Chemie, Weinheim, 1978.
Fig. 2. Proposed structures for the 1:1 +left) and 1:2 +right)
lanthanum glucuronic complexes.
[9] L.A. Krumholz, D.E. Goldberg, H. Boroughs, The Effects of
Atomic Radiation on Oceanography and Fisheries, Natl Acad.
Sci. Natl Res. Council Publ. No. 551 1957 +chap. 7).
[10] P.H. Brown, A.H. Rathje, R.D. Graham, D.E. Tribe, in: K.A.
Gschneidner Jr., L. Eyring +Eds.), Rare Earth in Biological
Systems in Handbook of the Physics and Chemistry of Rare
Earths, vol. 13, Elsevier/North Holland, Amsterdam, 1991.
[11] Martindale: The Extra Pharmacopoeia, 26th ed., Pharmaceu-
tical Press, London, 1972.
solution [18]. Interpretation of the pattern of the
stability constants shows that similarly to acetates,
the carboxylate group coordinates metal ion in
both LnL and LnL2 complexes. The complexation
is supplemented by the ring C+5) oxygen atom.
Moreover, the C+4) oxygen atom seems to play
an important role in the steric hindrance of
chelating ligand molecules. Because of the rela-
tively high coordination number of the lanthanide
cation +9 for the early lanthanides, decreasing to 8
for the last elements in the series [10,12]), it is to
be proposed that several water molecules ®ll the
vacant coordination sites of the cation. Proposed
structures are presented in Fig. 2.
[12] J.-C.G. BuÈnzli, G.R. Choppin +Eds.), Lanthanide Probes in
Life, Chemical and Earths Sciences Elsevier, Amsterdam,
1989.
[13] S. Balt, M.W. DeBolster, G. Visser-Luirink, Carbohydr. Res.
121 +1983) 1.
[14] T. Anthonsen, B. Larsen, O. Smisrúd, Acta Chem. Scand. 26
+1972) 2988.
[15] T. Anthonsen, B. Larsen, O. Smisrúd, Acta Chem. Scand. 27
+1973) 2671.
[16] Shugong-Zhang, Jiazuan-Ni, Wui Huaxue Xuebao, Chin. J.
Inorg. Chem. 6 +1990) 422.
The Fourier transform infrared spectra recorded
for both d-alduronic acids and their lanthanum+III)
crystalline salts in the region of 4000±150 cm21
con®rm the latter conclusions. Analysis of the
spectral range 820±870 cm21 allows to evidence
the presence of the b-anomer conformation in
free galacturonic acid as well as in both investi-
gated lanthanum salts. In addition, commercial
samples of the galacturonic acid are shown to
contain both anomers.
[17] C. Makridou, M. Cromer-Morin, J.-P. Scarff, Bull. Soc. Chim.
Fr. +1977) 59.
[18] L. Fuks, J.-C.G. BuÈnzli, Helv. Chim. Acta 76 +1993) 2992.
[19] S.N. Misra, T.N. Misra, R.C. Mehrotra, J. Inorg. Nucl. Chem.
25 +1963) 201.
[20] H.A. Tajmir-Riahi, Inorg. Chim. Acta 119 +1986) 227.
[21] H.A. Tajmir-Riahi, Carbohydr. Res. 125 +1984) 13.
[22] H.A. Tajmir-Riahi, Carbohydr. Res. 122 +1983) 241.
[23] M. Hineno, Carbohydr. Res. 56 +1977) 219.
[24] P.D. Vasco, J. Blackwell, J.L. Koening, Carbohydr. Res. 23
+1972) 407.
[25] R.A. Morgan, G.F. Richards, Acta Crystallogr. B 29 +1973)
2270.
[26] M. Sundaralingam, Biopolymers 6 +1968) 189.
[27] E.D. Atkins, I.A. Nieduszynski, H.H. Wills, W. Mackie, K.D.
Parker, E.E. Smolko, Biopolymers 12 +1973) 1865.
[28] E.D. Atkins, I.A. Nieduszynski, H.H. Wills, W. Mackie, K.D.
Parker, E.E. Smolko, Biopolymers 12 +1973) 1879.
References
Â
[1] H. Metivier, P. Gerasimo, P. Fritsch, R. Maase, Eulep. Newsl.
40 +1985) 28.

L. Fuks et al. / Journal of Molecular Structure 563±564 62001) 587±593
593
[29] K. Nakamoto, Infrared Spectra of Inorganic and Coordination
Compounds, Wiley, New York, 1963.
[32] W.J. Cook, C.E. Bugg, Biochim. Biophys. Acta 389 +1975)
428.
[30] L. DeLucas, C.E. Bugg, A. Terzis, R. Rivest, Carbohydr. Res.
41 +1975) 19.
[33] H.A. Tajmir-Riahi, Carbohydr. Res. 127 +1984) 1.
[34] G.E. Gurr, Acta Crystallogr. 16 +1966) 690.
[35] A. Terzis, Cryst. Struct. Commun. 7 +1978) 95.
[31] W.J. Cook, C.E. Bugg, Carbohydr. Res. 31 +1973) 265.